A communication apparatus or a microcomputer uses an oscillator to generate an accurate and stable reference clock. Patent Documents 1 and 2 disclose technologies that correct temporal changes in a crystal oscillator. Because the oscillator is expensive, it is desirable to avoid using the oscillator. A CR oscillation circuit is an alternative to the oscillator. The CR oscillation circuit is easily subject to changes in oscillation cycles due to temperature changes and power supply voltage changes. In particular, the CR oscillation circuit cannot be used as a reference clock for onboard devices used under an environment subject to large temperature changes.
For example, the local interconnect network (LIN), one of communication protocols for onboard LAN, uses a synch byte field 0x55 added to the header of a frame transmitted from a master node. A slave node counts the synch byte field using the reference clock to calculate an accurate value for one bit time and settle a communication rate (for example, see Patent Document 3). During detection of the synchronization signal, however, the temperature may change to vary the reference clock, making it impossible to correctly transmit or receive data.
To solve this problem, Patent Document 4 discloses a microcomputer that is controlled to ensure the constant data transmission time for a communication circuit without varying a time constant for the CR oscillation circuit when an oscillation frequency varies with the temperature. The microcomputer uses memory to store data for determining a communication rate CMR that ensures a constant data transmission time per frame managed by the communication circuit. The microcomputer reads the data stored in the memory in accordance with a detected temperature and assigns the determined communication rate CMR to the communication circuit. When a temperature range has insufficient data, the microcomputer uses a linear function to interpolate between upper and lower limits and generate an approximate communication rate CMR.    Patent Document 1: JP-A-2004-282118 corresponding to U.S. Pat. No. 7,228,117    Patent Document 2: JP-A-2002-305442    Patent Document 3: JP-A-2006-311237 corresponding to U.S. Patent Application Publication No. 2006/0245515    Patent Document 4: JP-A-2006-270917 corresponding to U.S. Pat. No. 7,356,719
A correction means described in Patent Document 4 needs to previously measure initial characteristics of individual products and write data corresponding to the characteristics to the memory. It is preferable to prepare as many data as possible for reduce errors. Increasing the amount of data also increases the number of processes for measuring and writing the initial characteristics and accordingly increases costs. As mentioned above, only a limited number of data is used for linear interpolation between the data. An error may easily result from initial characteristics for the temperature range to be interpolated. An error in the initial characteristics cannot be corrected during actual operations after shipment from a manufacturer. In addition, the CR oscillation circuit is subject to a gradual change in oscillation frequencies with the lapse of time under the same temperature or voltage condition. The correction means described in Patent Document 4 needs to rewrite the data so as to be able to correct an error due to the temporal change.
Thus, it is desired to provide the data reception apparatus and the microcomputer capable of generating a clock signal with an accurate frequency even when a CR oscillation circuit is subject to a temperature, voltage, or temporal change.